Botanical Journal of the Linnean Society (2001), 137: 127-137. With 8 figures
doi: 10.10os/boj1.2001.0458, available online a t http;//www.idealibrary.com on
I DE
8
Taxonomy of the western European endemic
Asparagus prostratus (A. officinalis subsp.
prostratus) (Asparagaceae)
W
Q. 0.N. KAY1, E. W. DAVIES2 and T. C. G. RICH3*
‘School of Biological Sciences, University of Wales, Swansea SA2 8PP and National Botanic Garden o f
Wales, Middleton Hall, Llanarthne, Carmarthenshire SA32 8HG
‘Prestons, Chewton Mendip, B a t h B A 3 4NU
3Department of Biodiversity and Systematic Biology, National Museum & Gallery of Wales, Cardiff
CFlO 3 N P
Received November 2000; accepted for publication February 2001
Asparagus prostratus Dumort., (wild asparagus) and A. officinalis L. (cultivated asparagus), often regarded as
subspecies or varieties of a single species, have been confused for historical and nomenclatural reasons. A taxonomic
review was carried out, and they were found to be distinct species which differ in morphology (characters retained
in cultivation), cytology, distribution and ecology, and they are reproductively isolated. The tetraploid A. prostratus
is unlikely to be the taxon from which the diploid A. officinalisevolved. Morphological descriptions are presented,
and A. prostratus is lectotypified. A. prostratus is a western European endemic of coasts of Belgium, Britain, the
0 2001 The Linnean Society of London
Channel Islands, France, Germany, Ireland, Spain and The Netherlands.
ADDITIONAL. KEY WORDS: cytology - infraspecific variation - lectotype - wild asparagus.
INTRODUCTION
Historically, Asparagus prostratus Dumort., wild asparagus and A. officinalis L., cultivated or garden
asparagus have been regarded as distinct species, or as
subspecies or varieties of a single species. Distinctions
between cultivated forms of Asparagus and the wild
populations that grow in maritime habitats on Atlantic
and western Mediterranean coasts have a long history.
For instance, Ray (1724) distinguished between “Asparagus . . . hortensis & pratensis . . . sativa Manured
Sparagus or Sperage, corrupte Sparrowgrass” and ‘Asparagus maritimus, crassiore folio”’. Broadly, the former corresponds to cultivars of A. officinalis(cultivated
asparagus) and the latter corresponds, a t least in
part, to the Atlantic-maritime A. prostratus and the
Mediterranean-maritime A. maritimus (L.) Mill. Botanists and gardeners who had seen the wild populations of Asparagus that are found in maritime
habitats on Atlantic and western Mediterranean coasts
had long been aware that they were very different in
appearance and character from the cultivated forms.
Corresponding author. E-mail: [email protected]
0024-4074/01/100127
+ 11 $35.00/0
Unfortunately there has been much confusion in
nomenclature and identification of the Asparagus species in this group. They make rather unsatisfactory
herbarium specimens as the growth form and coloration are lost and adequate collecting notes are rarely
provided. The specimens are often composed of side
branches rather than whole stems; the parts are fragile
and disintegrate upon handling. They are also hard to
illustrate satisfactorily. Many botanists have also paid
little attention to them as they considered that the
forms differed as a result of the differences in their
habitats. Despite cultivation experiments that repeatedly disproved this hypothesis (see below), this
view persisted and was particularly influential during
the nineteenth and early twentieth centuries. Apparent support was provided by the ease with which
cultivated asparagus was bird-sown in the wild, often
occurring on dunes and in other maritime habitats
well away from cultivated land and sometimes in association with the wild species, both of which are
variable. This led to much confusion. Further confusion
was caused by the long-continuing failure to realize
that the Mediterranean A. maritimus differed from the
Atlantic A. prostratus. Atlantic maritime populations
127
0 2001 The Linnean Society of London
128 Q. 0. N. KAY ET AL.
were regularly ascribed to A. maritimus, either as a
variety or subspecies of A. officinalis or as a species,
well into the twentieth century (e.g. Webb, 1944), and
A. maritimus was often thought t o be a synonym of A.
officinalis (e.g. Thielens, 1868; Wolley-Dod, 1937).
Asparagus prostratus was first described as a species
from plants growing on the coasts of Belgium and The
Netherlands by the Belgian botanist B.-C. J.Dumortier
(Dumortier, 1827). His epithet prostratus and short
description draw attention to its prostrate habit, which
is the most immediately obvious difference from A.
officinalis. Crepin (1864) claimed that ‘A. prostratus’
was only a form of A. officinalis modified by habitat,
but Dumortier (1869) thought that it was likely that
the plant seen by Crepin was naturalized A. officinalis
which was already common on dunes in the sites where
A. prostratus was said to grow (Crepin’s herbarium in
BR does not clarify this further), and he emphasized
that the prostrate habit and other distinctive characters of A. prostratus had maintained themselves in
garden cultivation for more than thirty years. Fortunately, A. prostratus was subsequently refound at
Mariakerke on the Belgian coast, and Thielens (1868)
published a description and excellent illustration of
this plant together with a defence of its status as a
species that was strikingly different from A. officinalis.
Unfortunately, Thielens also put forward the view that
the naturalized A. officinalis that grew on Belgian
dunes could be correctly described as A. officinalis var.
maritimus L. (i.e. A. maritimus).
We have each independently reviewed the taxonomy
of A. prostratus, and have come to the same conclusion
that it should be treated as a species distinct from A.
officinalis. E. W. Davies (now E. W. Woodward) undertook postdoctoral research at the University of Bristo1 in the late 1950s with the assistance from two
students, J. Roseweir and F. M. Hetherington, but a
serious riding accident prevented completion of the
work and only a few preliminary accounts were published(Davies & Willis, 1959;Davies, 1961a,b).&. 0.N.
Kay and R. F. John carried out genetic and cytological
studies on British material of A. prostratus, including
the relationship to A. officinalis, and reviewed its
conservation and ecology in reports to the Countryside
Council for Wales (Kay &John, 1995; Kay, 1997). L. K.
Rich (nee Wilkinson) and T. C. G . Rich have collated
data on its distribution and phytosociology in Europe
for conservation and on its variation for an infraspecific
flora project. This joint paper summarizes the taxonomic work; research on its ecology, distribution and
conservation in the British Isles will be published
elsewhere.
MORPHOLOGY
Characters used to distinguish A. prostratus and A.
officinalis by Warburg (1962) and Valdes (1980) are
listed in Table 1, and were used as the basis for further
morphological investigation. A total of 101 herbarium
specimens of A. prostratus and 76 of A. officinalis from
BR, L, NMW and the Centre Nationale Botanique de
Bailleul herbarium (herbaria abbreviations following
Holmgren, Holmgren & Barnett, 1990) representing
material from throughout Europe were measured in
detail, and supplemented with observations of herbarium material in B, BEL, BM, BMH, BRISTM, BTN,
C, CGE, DBN, DZS, K, P, RNG, S and TCD and field
observations in Britain, France, Ireland, The Netherlands and Spain. Data are plotted as relative frequencies t o compensate for the imbalance in the
numbers of specimens examined. Figure 1 shows the
measurements made, and male and female flowers.
Descriptions of stem height and form are predominantly based on field observations as they are
poorly represented on herbarium sheets. In the field,
the stems of A. officinalis are erect, much taller and
straight, although they may be sinuous above. Large
plants have many stems which may be up t o about
3 cm (rarely more) in diameter at the base. The stems
of A. pmstratus are more variable in habit, tend to be
thinner, curved and/or sinuous at least below, and tend
t o be fewer in number (rarely more than four). In
exposed situations the larger stems are usually prostrate or weakly ascending, whilst smaller ones may
vary from prostrate to erect. In sheltered or shaded
situations (such as amongst bushes) the stems may be
erect at the base with the tops usually spread sideways
(this is particularly marked in the dunes near Wassenaar in The Netherlands). Seedlings and young
plants of A. prostratus may have prostrate to ferect
stems. The branches and cladodes tend t o be held erect
to ascending in A. officinalis and ascending t o patent
in A. prostratus, but this is variable.
In both species the internode length is quite variable
depending on growth conditions, and adjacent internodes can vary markedly in length as lateral branching is not regular and in shaded conditions some side
branches may not develop. Mean lengths were calculated for the lowest ten internodes on each stem
(Fig. 1A).
In both species, cladodes occur in clusters of c. 4-15
on the main stem and branches, and vary markedly
in length within each cluster. The longest cladode (Fig.
1A) on up to ten branches was measured and averages
calculated. The length is quite variable even on the
same plank the lower cladodes of the lower side
branches are often the longest. Mean cladode length is
correlated with mean internode length in A. prostratus
(r=0.38, N=66, P ~ 0 . 0 0 5 but
)
not in A. officinalis
(r=0.12, N=58, BO.1) possibly due to the greater
influence of environmental conditions on the growth
of A. prostratus. There is at least one clone of A.
officinalis with very short cladodes but long internodes
TAXONOMY OF ASPARAGUS PROSTRATUS
129
Table 1. Characters which have been used to distinguish Asparagus prostratus and A. officinalis based on those given
by Warburg (1962) and Valdes (1980) (see descriptions for revised values)
Character
A. pmstratus
Stems
10-30(-100) cm, decumbent, with short internodes 3C200 cm, erect, with long internodes
(5-)10-25(-30) mm, often flexuous, green
(4-)5-lo(-15) mm, thick, rigid, glaucous
Cladodes
Pedicels
Flowers
Male flowers
Female flowers
Fruits
Seeds per fruit
Ploidy
2 4 - 8 ) mm
Often mixed with cladodes
7-8.5 mm, yellow, red-tinged at base
4 4 . 5 mm, yellow to whitish-green
5-7 mm
1-6, most frequently 2
2n = 40
(possibly referable to var. sulcatifolius Sennen & M. K.
Elias).
Pedicel length was measured from the stem to the
base of the hypanthium (including the stipe) on up to
ten open flowers/fruits on each plant (Fig. 1).Pedicels
on the main stem were usually longer than those on
side branches and male pedicels may be longer than
female pedicels. It is probable that pedicels continue
to grow slightly as fruits mature. Mean pedicel length
is highly correlated with mean internode length ( r =
0.508, N=44, P<O.OOl) in A. prostratus but not in A.
officinalis( r= 0.204, N = 36, D O . l), and is independent
of mean cladode length in both species ( r = -0.02, N =
58, P>O.1 and r=0.09, N=46, -0.1 respectively).
Pedicels may be patent to ascending on both species.
Both species are generally dioecious, but male A,
officinalis may occasionally have a few hermaphrodite
flowers (Anon, 1932, 1969). Perianth segments were
measured from the base of the hypanthium to the tip
of the longest perianth segment (Fig. lB,C) on up to
ten dried flowers (these are likely to have shrunk
in size compared with fresh flowers). Relatively few
samples were available, especially of female flowers
(this may reflect female plants being generally less
common than males in the field). Male flowers are
bigger than female flowers in both species. Mean male
perianth size is not correlated with mean internode
length, mean cladode length or mean pedicel length
in either species (all D0.05 or BO.1). In general, A.
prostratus is much more floriferous than A. officinalis
and tends to have many more flowers clustered on
the side branches amongst the cladodes, whereas A.
officinalis tends to have fewer flowers, mainly on the
barer parts of the branches and stem. However, this
is variable in both species.
Berry size (Fig. 1D) cannot be measured on pressed
fruits, and data were available for only eight fresh
collections of A. prostratus and four of A. officinalis.
Both Thielens ( 1868) and Dumortier (1869) considered
that the fruits of A . prostratus were obviously larger
A. officinalis
( 6 . 5 )1&20(-25) mm
Rarely mixed with cladodes
5 4 mm, yellow
c. 4 mm, yellow-green
7.5-8.5mm
5-6
2n = 20
than those of A. officinalis; Thielens stated that this
character on its own could be used to distinguish the
species (". . . le fruit, qui est deux a trois fois plus gros
que cela de l'officinalis. La grosseur des fruits suffirait
seule pour faire distinguer les d e w especes"), and
Dumortier similarly described the fruits of A. p m stratus as being twice as large as those of A. officinalis
('Les fruits sont deux fois plus gros"). In contrast, our
data indicate that A. officinalis generally has larger
fruits than A. prostratus, although we found large
overlap in the ranges. Fruit size is also dependent
on the number of seeds per berry (r=0.558, N = 146,
P<O.OOl for A. prostratus, r=0.685, N=90, P<O.001
for A. officinalis).
The number of seeds per berry is variable, possibly
partly due t o variable pollination. On average A. officinalis has 4.4 seeds per berry (all berries included),
nearly twice as many as A. prostratus (2.5 seeds per
berry), but there is again large variation between
individuals suggesting some genetic basis. There are
no significant differences in seed size.
Both species can be quite variable in morphology,
but although the quantitative characters show overlap
there is usually little trouble distinguishing the species
except for A. officinalis with small cladodes. It is easier
to identify fresh material in the field than dried material in the herbarium, and it is easier to tell the
species apart earlier in the season than in the autumn.
Histograms showing variation in mean internode
lengths and mean longest cladode length are given in
Figures 2 and 3. In both cases there is overlap between
the taxa which it is possible to relate at least in
part to environmental conditions. Small plants of A.
officinalis with shorter internodes and cladodes tend
to occur in exposed sites, whilst large plants of A.
prostratus with longer internodes and cladodes occur
in sheltered or partly shaded sites, especially on the
edge of scrub. Similarly there is overlap in the size of
mean pedicel, male perianth and female perianth
length (Figs 4-6), though these characters would be
130
Q. 0. N. KAY ET AL.
lq
W
I
7
k
Ii
1
Figure 1. Measurements of Asparagus. A, side branch from main stem of A. officinalis. B, female flower of A.
prostratus with nearest perianth segments removed to show ovary. C, male flower of A. prostratus with nearest perianth
segments removed to show stamens, vestigial ovary and out-curving perianth segments. D, fruit of A. prostratus.
Measurements are indicated as follows: c, longest cladode on branch. i, internode on main stem. 1, fruit length. p,
pedicel length (including stipe). q, perianth length. w, fruit width. Scale bars are 1cm in all cases.
TAXONOMY OF ASPARAGUS PROSTRATUS
2
131
0.1
$ 0.08
& 0.06
.-? 0.04
-2
a
P
0.02
;
n
"
6
9
3
12 15 18 21 24 27 30 33
Mean internode length (mm)
Figure 2. Variation in mean internode length (means of
up to 10 internodes from base of stem). (0)
Asparagus
officinalis(N=59); (m) A. prostrutus ( N = 6 8 ) .
a
&
3.5
4
4.5
5
5.5
Mean female perianth length (mm)
Figure 6. Variation in mean female perianth length
Asparagus officinalis
(means of up to 10 perianths). (0)
(N=18); (m) A. prostratus (N=22).
0.15
g 0.1
.2 0.05
3 5 7 9 11 13 15 17 19 21 23 25 27 29 31
Mean cladode length (mm)
Figure 3. Variation in mean longest cladode length
(means of up t o 10 longest cladodes from different
branches). (0)
Asparagus officinalis ( N = 71); (m) A. prostratus (N=92).
$
g
'
0.25 .0.2
0.15 ~
0
10
20
30
Mean cladode length (mm)
Figure 7. Separation of the taxa using mean cladode
length and mean perianth length for plants of each sex.
Asparagus prostratus: (4) male; (m) female. A. officinalis:
(A)male; ( x ) female.
CULTIVATION EXPERIMENTS
1L
7
8
9 10 11 12 13 14 15
Figure 4. Variation in mean pedicel length (means of
up t o 10 pedicels). (0)
Asparagus officinalis (N=46); (m)
A. prostratus (N=59).
4
4.5
5
5.5
6
6.5
7
Mean male perianth length (mm)
Dumortier (1869) reported that the prostrate habit of
A. prostratus was a fixed character which remained
Mean pedicel length (mm)
7.5
Figure 5. Variation in mean male perianth length
(means of up to 10 perianths). (0)
Asparagus officinalis
(N=24); (m) A. prostrutus (N=29).
less likely t o be influenced by the environment. When
the cladode length is plotted against perianth length
for each sex, there is good separation of the taxa (Fig.
7).
constant in garden cultivation and from seed, over a
period of over thirty years, and was not due to the effect
of persistent coastal winds; furthermore, A. officinalis
retained its erect habit even when growing close to
the sea. Further experiments by Lloyd (1897) and Des
Abbayes et al. (1971) on French material, by E. W.
Davies on Cornish and French material (unpublished
data), by Kay & John (1995) on British material, and
by T. C. G. & L. K. Rich on Spanish, Irish and French
material (unpublished data) show A. prostratus retains
most of its distinctive characters in cultivation under
sheltered conditions. Some of the reports to the contrary may be due to short term experiments; seedlings
of A. prostratus in greenhouses tend to grow erect to
ascending in their first year, but when planted out
in their second year tend to be more decumbent t o
ascending, and more prostrate still in their third year.
The few seedlings we have observed in the wild also
tend to be erect initially.
Size of internodes and cladodes, however, can be
affected by environment. A plant of A. prostratus collected from a very wind-exposed, droughted sandy
132
Q. 0. N. KAY ETAL.
promontory a t Pointe du Torche, Brittany, France had
2n =20 in 18 of the 19 cultivars that they studied, and
internodes 4 . 6 f 1 . 2 ~SD and cladodes 9.6mm
also in two wild populations from Turkey. However, a
+1.8mm SD long (N=8), but after five months in
genetically unusual old cultivar from southern Spain,
cultivation in a greenhouse produced new shoots with
‘VioletaHuetor’, proved to have 2n =40 (see also below).
internodes 15.1f 4 . 0 SD
Many investigators have reported producing tet~ and cladodes 1 6 . 9 ~
- 1.6 mm SD long ( N = 8; specimens in N M W ) . After
raploid and other polyploid cell lines, plants or clones
another year in cultivation outside in moderately exof A. officinalis, both by colchicine or other treatments
posed and droughted conditions the shoots had interdeliberately intended to induce chromosome doubling
~ and cladodes 8.1mmf2.0mm
nodes 7 . 3 k 3 . 1 SD
(e.g. Braak & Zeilinga, 1957; Ellison & Tiangco, 1970;
SD long (N=6).
McCollum, 1988) and as a result of the use of micropropagation techniques, especially callus or anther
culture (Reuther, 1983; Skiebe et al., 1991; Kohmura
CYTOLOGY
et al., 1995). Colchicine treatment has been found
to be most effective (Skiebe et al., 1991). Polyploids
ASPARAGUS PROSTRATUS
generated by micropropagation techniques are often
Chromosome counts of 2n = 40 have been reported for
unstable or genetically defective, but those produced
A. prostratus from the following sites:
by colchicine treatment can be sufficiently stable for
use in breeding work. Autotetraploid A. officinalisoften
Scheveningen dunes, Netherlands (Braak & Zeilshows agronomically desirable characteristics and atinga, 1957).
tempts have been made to introduce tetraploid (and
Kynance Cove, Cornwall (Davies & Willis, 1959;
even triploid, e.g. ‘Hiroshima Green’, see Kohmura et
F. M. Hetherington, unpublished data).
al., 1995) cultivars.
Culver Hole, Port Eynon, Glamorgan (Kay & John,
1995).
Pobbles Bay, Pennard, Glamorgan (Q. 0.N. Kay,
RELATIONSHIP OF A. PROSTRATUS TO
unpublished data).
A. OFFICINALJS AND OTHER TAXA
Cadgwith Cliffs, Cornwall (E. W. Davies, unpublished data; F. M. Hetherington, unpublished
It is often assumed that A. officinaliswas derived from
data; Kay & John, 1995).
A. prostratus but the difference in their chromosome
Western France (precise location uncertain, seed
numbers makes this unlikely, although they are clearly
via Nantes Botanic Garden) (McCollum, 1988).
related. It is also unlikely that a vegetable known to
Ballyteige Dunes, Ireland (J.P. Bailey & R. J. Gorthe ancient Greeks (Anon, 1932, 1969; Kidner, 1947)
nall, pers. comm., 1994; Q. 0.N. Kay, unpublished
was derived from a north-west European endemic. It
data).
would also appear that both species are related t o A.
maritimus and A. tenuifolius Lam. which we accept as
In addition, Davies & Willis (1959) reported a count
distinct
species following Valdes (1980).A chromosome
of 80 univalents seen in an apparently abnormal meicount of 2n =40 was reported for A. maritimus from
osis in a plant from Cadgwith. It seems probable
Italy (Bozzini, 1959), and 2n=20 for A. tenuifolius
that this was observed in a plant with a chromosome
Lam.
(Valdes, 1980). Further investigation of the cytonumber of 2n=40, not 2n=?80 as was implied by
logy
of
these species and of their relationships with A.
Warburg (1962). F. M. Hetherington also found a high
prostratus and A. officinalis is needed.
frequency of univalents (59.6Yo) and pollen sterility in
A wide-ranging survey of genetic variation in A.
tetraploid male A. prostratus from Cadgwith (unofficinalis was made by Geoffriau et al. (1992). They
published data).
studied 19 asparagus cultivars that are currently used
The breeding experiments by McCollum (1988) with
in
cultivation and breeding, and also two wild popuboth the tetraploid Nantes Botanic Garden accession
lations from Turkey. The modern asparagus cultivars
listed above and a second French accession of A. prothat are grown in The Netherlands, France, Germany,
stratus, from Plovan, Finistere, showed that this secBritain and the United States and Canada are thought
ond accession was also tetraploid, although no direct
to
have a very narrow genetic basis, all being derived
count was made.
from the eighteenth-century Dutch cultivar ‘Gewone
Hollandse’, later called ‘Purple Dutch. In contrast, in
ASPARAG US OFFICINALJS
Italy and Spain, and probably in some other Mediterranean countries, traditional populations (land
Many counts of 2n=20 have been reported for the
races) are thought to have been developed and grown
widespread cultivars of this species, for example by
in local areas. They thus included three Italian and
Bozzini (1959). The best comparative study of cultivars,
which included genetic comparisons, was that made
three Spanish traditional cultivars in their study. The
most interesting finding of their work, from the point
by Geoffriau, Denoue & Rameau (1992). They found
+
TAXONOMY OF ASPARAGUS PROSTRATUS
of view of studies of the relationships between cultivated A. officinalisand the wild maritime tetraploids
A. prostratus and A. maritimus, was that the Spanish
traditional cultivar ‘Violeta Huetor’, which is grown in
the Granada region of southern Spain, is tetraploid.
All the other populations that they studied were diploid. Their results show that ‘Violeta Huetor’ was well
differentiated from the other cultivars in its isozyme
patterns, although it only showed two unique bands,
and that it also showed much higher levels of variation.
The two ‘wild Turkish populations were also well
differentiated and variable, though quite dissimilar to
‘Violeta Huetor’. All the remaining cultivars showed
relatively low levels of isozyme variation, both within
and among cultivar populations, generally confirming
the traditional view of the narrow genetic base and
general genetic similarity of asparagus cultivars, despite the dioecious breeding system of the species
(which is still propagated almost entirely by seed). It
seems clear that ‘Violeta Huetor’ should be included
in any study of genetic relationships between A. officinalis,A. piostratus, A. maritimus and A. tenuifolius.
Its unusual isozyme patterns strongly suggest that it
is not simply a n autotetraploid derivative of a diploid
A. officinalis cultivar, and the possibility that it is
derived wholly or partly from A. maritimus, or even
from A. prostratus, should be borne in mind. An additional possibility that its discovery suggests is that
other local tetraploid land-races of cultivated asparagus, not necessarily derived from A. officinalis,
may exist, or have existed in the past. Isozyme studies
of A. prostratus, which have already indicated fairly
high levels of variation in British populations (Kay &
John, 1995), need to be extended to test its degree of
relationship to A. officinalis and to ‘Violeta Huetor’
and any other genetically unusual Asparagus cultivars
that may be found in the future. A. officinalis is widely
naturalized as a persistent escape from cultivation. Its
most likely region of origin has been suggested to be
eastern Europe or western Asia, probably in or near
the Caucasus (Sturtevant, 1890; Geoffriau et al., 1992).
A. prostratus and A. officinalis are genetically isolated from one another as a consequence of a strong
barrier to gene-flow caused by their difference in chromosome ploicly. Nehou (in des Abbayes et al., 1971)
reportedly failed to obtain any hybrids between A.
prostratus and A. officinalis, and no plant breeder has
reported obtaining any offspring from crosses between
tetraploid A. prostratus and diploid A. officinalis.Diploid-tetraploid crosses within A. officinalis (using experimentally produced tetraploid plants) normally fail
to set seed, and are in fact used by breeders as tests
of ploidy (McCollum, 1988). In a series of
diploid x autotetraploid crosses within A. officinalis
reported by Skiebe et al. (1991), 386 crosses from
diploid to tetraploid (male diploid t o female tetraploid
133
is presumably meant, although not stated) yielded only
two plants, both of which were tetraploid, while 1168
crosses from tetraploid to diploid yielded 42 triploid
and four tetraploid plants. These results suggest that
even in the absence of any other internal barriers (e.g.
cross-incompatibility, see Marcellan & Camadro, 1996)
gene-flow from diploid A. officinalis into tetraploid
A. prostratus via the direct production of tetraploid
zygotes (which, if they survived to produce plants,
might then be able to interbreed with normal tetraploid
A. prostratus) is likely to be very severely restricted
by a very low frequency of successful tetraploid zygote
formation. It should be remembered that as commercial
asparagus growers try to eliminate females from cropping fields because of their lower yield of shoots, transfer of male A. officinalis pollen to female A. prostratus
is likely to be overwhelmingly the most frequent in
situations where cross-pollination between a commercial asparagus crop and a wild stand of A. prostratus may occur. In the other direction, gene-flow via
the small percentage of viable triploid seeds produced
by diploid female A. officinalis parents (3.6% in Skiebe
et al.’s experimental A. officinalis 4x x 2x crosses) would
depend on the fertility of the triploid plants that grew
from these seeds, and especially on the frequency
with which they produce viable unreduced-triploid or
haploid gametes; this is likely to be extremely low or
even zero. There are no reports of the experimental
production of the triploid hybrid between A. prostratus
and A. officinalis, nor of its confirmation in nature
(interesting variable, mixed populations a t Wassenaar,
Netherlands, are currently under investigation). As it
would be expected to be of very low or zero fertility,
the best way to detect possible triploids in the field
would be to look for male plants of intermediate habit
with shrivelled anthers and defective pollen.
The only breeding work involving the production of
A. officinalis x A. prostratus hybrids that has been
reported is that of McCollum (1988). He obtained tetraploid hybrids from crosses between colchicine-induced tetraploids of A. officinalis cv. ‘Viking’ and two
accessions of tetraploid A. prostratus from western
France. His tetraploid F1 plants were of intermediate
morphology, and he describes them as fully fertile,
although he apparently obtained seeds only from the
hermaphrodites in the two F1 families that included
hermaphrodites, among 14 F1 families which were
otherwise entirely dioecious. Hermaphroditism is unknown in A. prostratus, and rare in A. officinalis, but
is known to occur in tetraploid lines of cv. ‘Viking’, so
was probably derived from the latter. These F2 seeds
were released as germplasm sources for use in plant
breeding work, but subsequent work a t Guelph University showed that they were weak and extremely
susceptible to rust, and they have not yet been utilized
(I? Banks, pers. comm., 1997).
134
Q. 0. N. KAY ET AL.
DISCUSSION
We believe that A. prostratus should be treated as a
species distinct from A. officinalis for the following
reasons:
(1) It is morphologically distinct (it has prostrate mature stems with shorter internodes and cladodes,
and larger pedicels and flowers).
(2) It has a distinct chromosome number (tetraploid).
(3) It is reproductively isolated from A. officinalis.
(4) It has a distinct distribution and ecology (restricted
to coastal habitats in western Europe), whereas A.
officinalis is thought to be originally native in
eastern Europe or western Asia, perhaps in or near
the Caucasus.
We therefore present a formal taxonomic treatment
here. A. prostratus is one of a small and select number
of plant species that are endemic to maritime habitats
on the western Atlantic coast of Europe which are
full, normally evolved, outbreeding species (another
example is Rumex rupestris Le Gall).
1. Asparagus officinalis L. Species Plantarum: 313,
1753.
=Asparagus officinalisL. subsp. officinalis.
=Asparagus officinalis L. var. altilis L., Species Plantarum 313, 1753.
=Asparagus officinalis L. var. campestris Syme, in
Sowerby & Smith, English Botany, 3rd ed.: 182, 1902.
=Asparagus altilis (L.) Asch., Flora der Provinz
Brandenburg: 730, 1859.
=Asparagus hortensis Mill. ex Baker, J. Linn. SOC.
Lond., 14: 598, 1875.
Vernacular names. Asparagus, cultivated asparagus,
garden asparagus.
Lectotype. Herb LINN. 434, 1 (LINN) (Valdes, 1975).
Description. Plant usually mid- to olive-green, often
shiny. Stems (40-)80-200cm, up to 3(-5)cm or more
in diameter at base, erect, straight (sometimessinuous
towards apex), branches usually ascending. Mean
internode length 12-36mm (range of all internodes
4-89mm). Mean length of longest cladodes
(6-)lo-32 mm (range of all cladodes 4.5-34 mm),
usually ascending and often curved, thin, flexible.
Mean pedicel length (6-)7-15 mm (range of all pedicels
5-17(-25) mm). Flowers usually spaced out, sometimes
mixed with cladodes on side branches. Mean male
perianth length 4.0-5.7(range of all male perianths 3.8-7 mm), yellow (rarely flushed pinkish), tips
straight. Mean female perianth length 3.14.1mm
(range of all female perianths 2.74.3mm), pale yellowish-green, tips straight. Mean berry width
6.7-7.7 mm diameter (range all berries 4.5-9 mm) with
mean 2.2-5.2 seeds (range 1-7). Flowers mainly June
to September.
Chromosome number. Normally 2n = 20 (dip1oid)s
rarely, some cultivars 2n =40 (tetraploid).
A highly polymorphic taxon, and no doubt more
variable than described above. The plants naturalized
in the wild are very variable between sites, perhaps
depending on which cultivar they were derived from
as well as modification by environmental conditions.
Distribution. Widely naturalized throughout Europe
and elsewhere in the world, occurring on waste ground,
railway lines, field margins, woodland edges, sand
dunes, shingle and exceptionally sea cliffs. The native
range is uncertain but it is thought t o be of eastern
European or western Asian origin.
2. Asparagus prostratus Dumort., Florula Belgica 138,
1827.
=Asparagus altilis (L.) Asch. var. prostratus (Dumort.)
K. Rcht., Plantae Europeae 1: 230, 1890.
=Asparagus maritimus auct., non L.
=Asparagus officinalisL. subsp. prostratus (Dumort.)
Corb., Nouvelle Flow de Normandie, 568, 1894.
=Asparagus officinalis L. subsp. prostratus (Dumort.)
E. F. Warb., Flora of the British Isles, 1221, 1952.
=Asparagus officinalis L. var. maritimus Sm., Flora
Britannica, 1: 369, 1804.
=Asparagus officinalisL. var. maritimus H. C. Hall,
Flora Belgii Septentrionalis: 278, 1825.
KEY TO A. PROSTMTUS AND A. OFFZCZNLZS
1.
1.
Plant usually green; stems erect; mean length of longest cladodes (5-)12-32mm (if less
than lOmm then mean internode length usually more than 15mm); mean pedicel length
(6-)7-15 mm ..................................................................................................
1.A. officinalis S.S.
Plant often glaucous; at least the largest mature stems usually prostrate, decumbent or
ascending (smaller supplementary stems of those in shaded or sheltered conditions often
erect); mean length of longest cladodes 2-16 mm; mean pedicel length 3-9(-10) mm ..............
................................................................................................................................ 2. A. prostratus
TAXONOMY OF ASPARAGUS PROSTRATUS
=Asparagus officinalis L. var. maritimus Gren. &
Godr., Flore ak France: 231, 1856.
=Asparagus officinalis L. var. pmstratus Asch. &
Graebn. Synopsis der Mitteleumpaischen Flora 3:
295, 1905-1907.
Vernacular name. Wild asparagus. We reject ‘sea asparagus’ to reduce confusion with A. maritimus.
Lectotype. Asparagus pmstratus was first described
as a species distinct from A. officinalis by Dumortier
(1827) “1815 prostratus: caule geniculato arcte humifuso; foliis fasciculatis Iinearibus; pedunculis geminatis diuegentibus apice articulatis, corn118
longioribus.. . . I n maritimus Flandriae et Hollandiae!”. The prostrate habit and larger corollas are
characters recognized as distinguishing the two taxa
today.
Dumortier’s herbarium is held in the National Botanical Garden of Belgium (BR), which was searched
for specimens from Belgium or The Netherlands likely
to have been seen by Dumortier in or prior to 1827
and which agreed with his description. There were
four sheets of A. pmstratus and three of A. officinalis
S.S. with determinations by Dumortier (the latter with
BR nos. 1 116 928, 1 116 929 and 810 987). These
collections, together with the original description and
the subsequent reiteration (Dumortier, 1869), demonstrate his understanding of the taxa with which we
agree.
The first sheet of A. pmstratus (BR no. 1 116 924)
is labelled “Asparagusprostratus, nob. Prodr. F1. Belg.
I n dunis Flandriae occidentalis. B. C. Du Mortier
1863”; it thus significantly post-dates the original description. The second sheet is from herb. F. Crepin (BR
no. 1 116 925), and is labelled “Asparagusprostratus,
nob. Pmdr. Fl. Belg. Cmit dans les dunes de Flandre,
pres Ostende, Mariakerke, Furnes et aussi en Holland2
with further annotations about its growth form; this
label is considered to post-date the original publication.
The third sheet (BR no. 810 380) is labelled ‘Dunes de
Scheueningue” by an unknown hand (not Dumortier)
with annotations by Dumortier “Asparaguspmstratus
in maritimus Flandriae” but is undated. The fourth
sheet (BR no. 1 116 927) is labelled “Asparagusp m stratus, nob., dunes de Flandrepres Ostende & Furnes”
in Dumortier’s handwriting and is clearly his new
plant, although it is undated; it is hereby selected as
the lectotype.
Description. Plant usually glaucous (especially when
exposed), sometimes green. Stems 10-70(-130) cm, at
least the largest usually prostrate, decumbent or ascending, rarely erect with tops curving sideways (smaller supplementary stems, or those in shade or sheltered
135
crevices more often erect), usually curved and/or sinuous, branches usually spreading to patent. Mean internode length 3-15(-20, exceptionally -50) mm (range
of all internodes 2-50 (exceptionally 150)mm). Mean
length of longest cladodes 2-16(-18) mm (range of
all cladodes 2-24 mm), usually ascending to patent,
straight, often stout and rigid. Mean pedicel length
3.2-10.4 mm (range of all pedicels 2-13 mm). Flowers
often abundant, mixed with cladodes on side branches.
Mean male perianth length 4.7-8.0mm (range of all
male perianths 4.2-8.3 mm), reddish or copper-flushed
yellow, out-curved a t tip. Mean female perianth length
3.5-5.9 nun (range of all female perianths 3.3-7.5 mm),
yellow to whitish-green, out-curved a t tips. Mean berry
width 5.3-7.9mm in diameter (range of all berries
4-lo(-11) mm) with mean 24(-5) seeds (range 1-6).
Flowering mainly May to July (-September).
Chromosome number. 2n = 40 (tetraploid).
A variable species between and sometimes within
sites. Table 2 summarizes variation in characters from
different countries. Plants from western France and
Wales (often in sandy soils) tend to have short internodes and cladodes, which may be partly related
to exposure. Plants from The Netherlands, cliffs in
Cornwall and northern Spain tend to have long internodes and cladodes. Plants from Jersey have characteristically short pedicels. Male perianth length is
more variable than female perianth length. Plants also
vary between sites within countries, and differences
are maintained in cultivation (Kay, 1997). Variation
within sites is partly related to soils and exposure, but
there may also be significant morphological variation
which is presumably a t least partly genetically determined (Kay & John, 1995). For instance on the
limestone headland a t Playa de Cuevas de Mar, Nueva,
Asturias, Spain plants varied from erect to nearly
prostrate, had branches ascending or spreading, and
were highly variable in the numbers of flowers produced (Rich & Rich, 1999; NMW).
Distribution. A western European endemic occurring
in Belgium, Channel Islands, England, Germany,
France, Ireland, Spain, The Netherlands and Wales
(Fig. 8).
It is locally abundant in Spain along the north coast
(Rich & Rich, 1998, 1999). It is widespread in most of
the sand-dune systems of the Atlantic coast of France,
from Biarritz on the Spanish frontier to southern Brittany (we have no localized records for the south of this
area so they are not presented in Fig. 7), and becomes
rare in the north of Finistere (F. Bioret pers. comm.,
1997) and along the English Channel (Provost, 1993;
Lemoine & Claustres, 1994). In Belgium it is now a t
best very rare (De Langhe et al., 1978) and we have
136
Q. 0. N. KAY ET AL.
Table 2. Variation in characters of Asparagus prostratus by country (insufficient data for Ireland). Figures are
means & standard deviation
Country
Mean internode Mean cladode
length (mm)
length (mm)
n
~~
Spain
France
Jersey
England (Cornwall)
Wales
Belgium
The Netherlands
32
8
3
11
7
17
9
Mean pedicel
length (mm)
Mean male
Mean female
perianth length perianth length
(mm)
(mm)
~~
12.4 k6.6
10.6k4.6
13.0f2.6
12.5+6.1
8.8k3.3
17.0f5.8
22.4+ 10.0
10.4f3.4
6.0 f 2.2
7.3 f 2.4
10.1k2.9
5.6k1.8
6.9k2.1
10.0k4.3
+
6.5 1.9
5.6 f1.9
3.7k0.7
5.1 1.3
6.351.5
7.4 f1.2
7.2f2.1
5.8 k 0.6
6.1 20.6
6.8
7.4 20.5
5.8
5.9 k 0.6
6.2
4.3 0.6
4.2 k0.5
4.1k0.5
4.5 f O . 8
4.1
5.0
extinct on Anglesey (Wilkinson, 1999). In Ireland it
occurs in a few sites in the south-east from Waterford
to Wicklow.
There are also possible specimens from the Baltic
coast on maritime sands a t Warnemunde, Rostock,
Germany (L) and from the small island of Nyord, near
Ulfshale on the east coast of Denmark (C) which remain
to be confirmed.
Figure 8. Distribution map of Asparagus prostratus (all
dates). It is stated to be frequent on the coats of SW
France (cf. Lemoine & Claustres, 1994) although we have
traced no localized records, and its occurrence on the
Baltic coasts has not been confirmed (see text).
not been able to trace any recent records. In The
Netherlands there are over 20 sites on dunes but it
has been lost from several former outlying localities,
with post-1950 Atlas records from only one site on the
northern island of Schiermonnikoog, one on Walcheren
in the south, and two near the mouth of the Rhine
(Mennema, Quene-Boterenbrod & Plate, 1985). In Germany it has been confirmed only from the island of
Borkum, Ostfriesische Inseln; Haeupler & Schonfelder
(1989) do not indicate its recent occurrence. In England
it is very locally distributed in the south-west, but is
extinct in Sussex, and in Wales it is still extant in four
out of five known sites in South Wales but apparently
Ecology. It occurs in a range of coastal habitats, predominantly on rocky cliffs, in coastal grasslands and
scrub, and on sand dunes, and more rarely on shingle
and exceptionally a t the saltmarsh interface. It usually
occurs within 100 m of the sea (rarely up to 200m,
and exceptionally in The Netherlands 3km inland)
from &sea level to c. 50 m altitude. It appears to be
commonest on calcareous soils (sand dunes, limestone
crevices) but also occurs on sandstones (perhaps where
soil pH is influenced by salt spray). Festucu rubru L.
is a n almost constant associate throughout its range
(Davies, 1961a; Rich & Rich, unpublished data).
ACKNOWLEDGEMENTS
We are grateful to the Keepers of the herbaria for
access to collections and libraries, and to the following
people for their generous help, advice and guidance:
Carlos Aedo, Jacques van Alphen, John Bailey, Joan
Banks, Paul Banks, Frederic Bioret, Brian Bonnard,
Humphrey Bowen, Nigel Brown, Jill Cowley, Ruth
Davis, Leni Duistennaat, Stephen Evans, M. Fernandez-Carvajal, Rosemary Fitzgerald, Rhona Floate,
Colin French, Ernie Goins, Richard Gornall, George
Hutchinson, Rosemary John, Andy Jones, Stephen
Jury, Andrew Lack, Barbara Lowrie, Andrew Malloch,
Serena Marner, Mike Martin, Len Margetts, Mike
Nichols, David Painter, Alan Paton, David Pearman,
Chris Preston, Dick Roberts, Ronald Rutherford, David
Stevens, Alison Stewart and Arthur Willis. We would
particularly like to thank Lindi Rich for stimulating
TAXONOMY OF ASPARAGUS PROSTRATUS
Q.O.N.K.’s and T.C.G.R.’s interest in wild asparagus.
Work by E.W.W. was f u n d e d by a scholarship from
Bristol Zoo. Work by Q.O.N.K. was part funded by the
Countryside Council for Wales.
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